1. Field of the Invention
The present invention relates to an improved technique related to a heater mechanism for maintaining at a predetermined temperature a material melt within a crucible of a crystal pulling apparatus.
2. Description of the Related Art
Conventionally, there has been known a crystal pulling apparatus wherein a seed crystal is brought into contact with, for example, a melt of silicon (semiconductor material) and is subsequently pulled while the seed crystal is gradually rotated, to thereby grow a single crystal. In such a crystal pulling apparatus, a heater mechanism is disposed around a crucible in which a silicon melt is contained, so as to maintain the silicon melt at a predetermined temperature. Among such heater mechanisms, there has been known a heater mechanism which can be moved vertically so as to vary the temperature of a melt when, for example, it is necessary to control the diameter of a crystal.
FIG. 5 shows an example of such a vertically movable heater. In FIG. 5, a heater mechanism 51 includes a cylindrical graphite heater 52 which surrounds an unillustrated crucible, and an electrode 53 made of copper or the like and connected to the lower portion of the graphite heater 52. The electrode 53 is passed through an electrode insertion hole h formed in a melt spill tray 54, which is disposed under the crucible, so that the heater mechanism 51 can move vertically. However, since the electrode 53 is projected upward from the melt spill tray 54 into a furnace as shown in FIG. 5, the vertically movable heater suffers the following significant drawbacks. For example, if a melt should leak from a crucible due to damage to the crucible, the leaking melt would flow along the heater 52 and reach the electrode 53, or splash onto the electrode 53. In such a case, the electrode 53 made of copper or the like would melt within a short period of time. This may cause a leak of the internal cooling water, resulting in the risk of a serious accident such as a steam explosion. Therefore, it has been desired to solve the above-described drawbacks.